R1:846f01e221a9

Correct self-review check in Differential

Summary: This check is currently wrong -- the actor is only //coincidentally// the owner (and only most of the time). It also raises at parse time, preventing any user from parsing a message with their own name in the "Reviewers" field. Instead, check against the right owner PHID and raise it only if a revision is available. See https://github.com/facebook/arcanist/issues/54 and next diff.

Test Plan: Tried to add myself as a reviewer to revisions I own via web…